Electromagnetic radiation in the terahertz range have demonstrated great potential in the imaging applications for biomedicine, security, and industrial quality control, due to its high spatial resolution (compared to millimeter wave) and non-ionizing natures (compared to X-ray). At present, the barrier to the wide application of this emerging sensing technology is mainly due to the difficulty of the high-power signal generation. Conventional THz sources include quantum-cascade lasers (“QCL”), photoconductive emitters, vacuum electronics, and III-V Schottky diode multiplier chains. However, these solutions have significant drawbacks, such as the high cost, large form factor, and stringent operation conditions (e.g., cryogenic cooling for QCL). Because of these, active THz imaging microsystems using integrated circuit technology are drawing increasing attention. In particular, imagers based on CMOS and BiCMOS processes are expected to not only resolve the above problems, but also achieve a high systematic integration level and high yield. This enables portable THz imaging equipment with a low cost.
However, there are several challenges towards this goal. First, the radiated power of existing THz transmitters is still insufficient. This is mainly due to the limited speed and breakdown voltage of the silicon transistors. The first THz CMOS radiator source reported in 2008 only generated 20 nW power at 410 GHz. Since then, significant progress has been made with synergistic efforts in device, circuit, and electromagnetism. A 390 μW power was previously obtained in the 288 GHz radiator based on a triple-push oscillator topology. A known 338 GHz phased array achieved 810 μW power. A known self-feeding oscillator array generated 1.1 mW radiated power at 260 GHz. Besides this work in CMOS, radiation sources in BiCMOS processes also demonstrate great potential, thanks to the superior speed and breakdown voltage of the SiGe heterojunction bipolar transistor (“HBT”). For example, radiators using a 130 nm SiGe BiCMOS process (fmax=500 GHz, VCEO=1.6 V) achieve 1.3 mW of power at 245 GHz and 74 μW (single element)/1 mW (incoherent array) of power at 530 GHz. To some extent, larger total radiated power can be obtained through the combination of an increased number of array elements. By comparison, the DC to THz radiation efficiency is more relevant to the performance of the devices and basic circuit blocks. It is particularly important for energy and thermal limited portable systems. Within less than a decade, the DC to THz radiation efficiency of silicon sources has increased by over 1000×. However, due to the approach of harmonic generation, the absolute efficiency level is still low. Previously reported highest DC to THz radiation efficiencies are 0.14% in CMOS and 0.33% in SiGe BiCMOS.
The challenge of the on-chip active THz imaging system also resides in the receiver side. Due to the lack of power amplification for THz signals (fin>fmax), focal-plane arrays in silicon rely on the direct passive detection using nonlinear devices, such as Schottky diode and MOSFET. This leads to limited sensitivity and further requires high-power generation from the transmitter. On the other hand, due to the Rayleigh diffraction limit and the usage of resonant antenna coupling, the size of an imaging pixel at THz, especially at low-THz (˜300 GHz) is large. It is therefore difficult to accommodate a large number of pixels on a single silicon die. Therefore, mechanical scanning is commonly used, which unfortunately prohibits the miniaturization of the imager and leads to long imaging time. To solve this issue, capability of electronic beam scanning is highly desired.
Non-ionizing terahertz imaging using solid-state integrated electronics has been gaining increasing attention over the past few years. However, there are currently several factors that deter the implementations of fully-integrated imaging systems. Due to the lack of low-noise amplification above fmax, the sensitivity of THz pixels on silicon cannot match that of its millimeter-wave or light-wave counterparts. This, combined with the focal-plane array configuration adopted by previous sensors, requires exceedingly large power for the illumination sources. Previous works on silicon have demonstrated 1 mW radiation; but higher power, as well as energy efficiency, are needed for a practical imaging system. In addition, a heterodyne imaging scheme was demonstrated to be very effective in enhancing detection sensitivity. Due to the preservation of phase information, it also enables digital beam forming with a small number of receiver units. This however requires phase locking between the THz source and receiver LO with a small frequency offset (IF<1 GHz). Although a 300 GHz PLL was reported with 40 μW probed power, on-chip phase locking of high-power THz radiation remains very challenging.
Generally, to maximize the harmonic (2f0) output power inside a radiating oscillator, it is advantageous to (i) achieve the optimum voltage gain of the transistor at f0 to maximize the oscillation activity, (ii) isolate the base and collector at the harmonic to eliminate the self-power-cancellation/loading effects, (iii) decouple the base and collector at DC for optimum biasing, and (iv) efficiently radiate the harmonic signal without long, lossy feed lines (used for resonance at f0 in previous works). Unfortunately, none of the previous topologies can simultaneously meet these conditions.